The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve can be seen in FIG. 1. They are low coercivity (free) ferromagnetic layer 11, non-magnetic spacer layer 12, magnetically pinned layer 13 and magnetic pinning layer 14 (generally an antiferromagnetic material). Also seen in the figure are lower and upper conductive leads 15 and 16 respectively. In practice there would also be a capping layer (not shown) directly above the free layer and upper and lower magnetic shields (shown as 61 and 62 respectively in FIG. 6).
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, dictated by the minimum energy state, which is determined by the crystalline and shape anisotropy, current field, coupling field, and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
First generation GMR devices were designed so as to measure the resistance of the free layer for current flowing in the plane (CIP) of the film. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) are also being developed. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack.
Although the layers enumerated above are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with these structures. Magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide a device structure conducive to ensuring that the free layer is a single domain so that the domain configuration remains unperturbed after fabrication and under normal operation.
This is readily accomplished in a CIP device by placing permanent magnets on either side of the GMR stack. These abut the free layer and ensure that it remains a single domain at all times. Since, in the CIP design, sensing current flows along the line connecting the bias magnets, any sensing current that gets shunted into them can still be directed into the leads and thus be detected, so the bias magnets can be placed close together without affecting the read width of the device.
In the case of a CPP device it is much more difficult to establish longitudinal magnetic bias. Bias by in-stack magnets is constrained by the stack thickness and suffers from conflicts between the longitudinal and the transverse biases. Biasing schemes used for CIP GMR devices were previously thought to be impractical for CPP devices due to current shunting by the abutted hard magnets.
The present invention provides a solution to this problem.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,560,077, Fujiwara et al. disclose a conducting part having an area smaller than an area of the free layer while Pang et al. show lead structures abutting a sensor layer in U.S. Pat. No. 6,496,334. Yuan et al. (U.S. Pat. No. 5,739,987) describe a lead that defines an active read track width while Beach teaches two hard magnets, one on either side of a free layer and a lead on an AFM layer, in US 2002/0131215.